In the art of fabricating optical fibers, it is known to fabricate preforms whose doping is varied in the radial direction as a function of the required index profile in the fiber, and then to draw the preform to obtain the optical fiber. One conventional preform fabrication method consists of successively depositing layers of silica by modified chemical vapor deposition (MCVD) into a deposit tube to form a preform core and then to form a sleeve around the deposit tube; the sleeve is generally deposited onto the preform in such a way that the outside diameter of the preform is constant. Other fabrication techniques include outside vapor deposition (OVD) and vapor axial deposition (VAD). The resulting preform is then drawn.
For applications in optical transmission systems it is desirable for the propagation characteristics of the fiber, for example its chromatic dispersion, to be closely controlled along the fiber. Dispersion can be kept constant or it can be varied in a controlled manner between two opposite values, as proposed for example in EP-A-0 737 873. The propagation characteristics are a function of the index profile of the fiber, characterized by the refractive indices and radii of the layers deposited by MCVD, OVD, or VAD. Despite the great care taken in fabricating the preform, it is possible for the preform to suffer from defects and to be less than perfectly homogeneous, or more generally not to conform perfectly to its design values.
Generally speaking, the expression “core radius of a fiber” means the value of the radius beyond which variations in the index or the radius relative to its design value has minimal influence on the propagation characteristics of the fiber. In other words, it refers to the outer limit of the profile which defines the propagation characteristics of the fiber. For example, for a “stepped index” profile, the core radius is the outside radius of the index step. For a trapezium and ring profile the core radius is the outside radius of the ring. The above definition is transposed to a preform on the principle of geometrical scaling. It is important to note that the core radius of a preform defined in this way can be significantly different from the inside radius of the deposit tube.
Examples of the above defects are variations along the preform departing from respective nominal values, in the diameter of the core of the preform, in the radii of the deposited layers, or in the outside diameter of the preform. The defects can also be variations along the preform in the indices of the various layers of the preform departing from their design indices. For example, a 1% to 2% radial increase in the diameter of the core of the preform for a flat chromatic dispersion slope fiber can cause variations in chromatic dispersion of 1 picosecond per nanometer-kilometer (ps/(nm.km)) to 2 ps/(nm.km) at 1550 nm; such variations reduce the production yield. For dispersion compensated fibers (DCF) and reverse dispersion fibers (RDF) the variations in the characteristics of the preform can induce variations in dispersion of up to 10 ps/(nm.km) at 1550 nm.
WO-A-98 25 861 describes a fiber for soliton transmission applications, in which the outside diameter of the fiber varies. It states that dispersion varies with the diameter of the fiber. In a first embodiment proposed in that document a preform is formed by depositing layers having thickness that increases from one end of the preform to the other. The resulting conical core of the preform is surrounded with its cladding before the fiber is drawn. In a second embodiment described in the document a preform is formed by depositing layers of constant thickness but with the proportions of dopant varying. In either case, drawing the preform produces a fiber of constant diameter whose propagation characteristics vary as a function of position along the fiber.